从 SI 到 ImperialSystem 的转换工作正常，但相反的工作不起作用。 错误信息： static_cast: cannot convert from ImperialSystem to SI

代码：

#include
#define endl '\n'
using std::cout;
#define MTRTOFEETRATIO 3.28084;

/*
Write two classes to store distances in meter-centimeter and feet-inch systems respectively. Write conversions functions so that the program can convert
objects of both types.
*/
class SI;
class ImperialSystem {
private:
    int mfeet;
    int minch;
public:
    ImperialSystem(int m, int cm) :mfeet{ m }, minch{ cm }{};
    ImperialSystem(float dis) :mfeet{ static_cast(dis) }, minch{ static_cast((dis - mfeet) * 12) } {}
    operator float() {
        return mfeet + minch / 12.0;
    }
    operator SI();
    friend std::ostream& operator <<(std::ostream& out, const ImperialSystem& dis);
};

class SI {
private:
    int mmeter;
    int mcentimeter;
public:
    SI(int m, int cm) :mmeter{ m }, mcentimeter{ cm }{};
    SI(float dis) :mmeter{ static_cast(dis) }, mcentimeter{ static_cast((dis - mmeter) * 12) } {}
    operator ImperialSystem();
    friend std::ostream& operator <<(std::ostream& out, const SI& dis);
};

std::ostream& operator <<(std::ostream& out, const SI& dis) {
    out << " " << dis.mmeter << " m " << dis.mcentimeter << " cm ";
    return out;
}
std::ostream& operator <<(std::ostream& out, const ImperialSystem& dis) {
    out << " " << dis.mfeet << " ft " << dis.minch << " in ";
    return out;
}
ImperialSystem::operator SI() {
    double feet = mfeet + minch / 12;
    double meter = feet / MTRTOFEETRATIO;
    return meter;
}
SI::operator ImperialSystem() {
    double meter = mmeter + mcentimeter / 100.0;
    double feet = meter * MTRTOFEETRATIO;
    return feet;
}


int main() {
    SI s{ 20,35 };
    cout << s << " =  " << static_cast(s) << endl;//this works
    ImperialSystem i{ 10,11 };
    cout << i << " =  " << static_cast(i) << endl;//but this doesnot

    return 0;
}